The present invention relates to noise elimination for FM demodulators adapted to demodulate intermittent frequency-modulated waves and, more particularly, to noise elimination of this kind designed to eliminate click noise in the demodulated signal that is caused by abrupt changes of D.C. components in the received carrier wave.
In some frequency-modulation communication systems, the carrier wave is transmitted, to save transmission energy, only when the transmission signal (for instance, a speech signal) is present. The output of an FM demodulator in such a communication system having an intermittent carrier wave tends to include fluctuating D.C. components, the fluctuation depending upon the presence and the absence of the carrier. Such fluctuation is attributed to the deviation of the center frequency of the FM demodulator from the carrier wave frequency. Such frequency deviation is unavoidable to a certain extent. When the carrier wave has been turned ON from its OFF state, the D.C. component of the demodulated output is changed abruptly to cause a click noise due to transient response of subsequent circuits. In one approach for eliminating this click noise, the demodulated signal is usually cut off until the click noise is sufficiently attenuated. This method, however, involved the problem of the front-end mutilation of a speech signal.